1. Field of the Invention
The present invention relates to a booster type switching regulator (herein after referred to as a booster type SW regulator) in which an inrush current for charging an output capacitor from the power supply through a coil can be suppressed when a power supply is turned on.
2. Description of the Related Art
A conventional booster type SW regulator has a structure such as that of the circuit shown in FIG. 6.
An input power supply 20 of FIG. 6 is connected to a coil 21 and a power supply terminal 1 of a booster type SW regulator control circuit 30, the other end of the coil 21 is connected to a drain of a switch (xe2x80x9cSWxe2x80x9d) element 22 and an anode of a commutation diode 23, a cathode of the diode 23 is connected to an output voltage terminal 2 of the booster type SW regulator control circuit 30, and a capacitor 24 and a load 25 are connected to the output voltage terminal 2. When the voltage of the output voltage terminal 2 is denoted by Vout, the booster type SW regulator control circuit 30 controls the ON/OFF state of the SW element 22 so that Vout becomes constant. A gate of the SW element 22 is connected to a terminal 3 of a driving circuit 31 for the SW element, and is driven by a voltage Vext of the terminal 3 to turn ON/OFF the SW element 22. In FIG. 6, the SW element is an N-ch MOS transistor, and in order to turn ON the SW element, the voltage Vext of the output terminal 3 of the driving circuit 31 becomes a positive voltage xe2x80x9cHxe2x80x9d, and in order to turn OFF the SW element 2, a GND level voltage is outputted as the voltage Vext. Both a source of the SW element 22 and a substrate are connected to the GND level.
A switch circuit (hereinafter referred to as a SW circuit) 10 is for connecting the power supply 20 and the booster type SW regulator circuit, and when the booster type SW regulator circuit is in an OFF state, the SW circuit 10 is opened (hereinafter, a state where the SW circuit 10 is opened will be referred to as a state where the SW circuit 10 is in an OFF state). When the booster type SW regulator circuit is turned ON, the SW circuit 10 is closed (hereinafter, a state where the SW circuit 10 is closed will be referred to as a state where the SW circuit 10 is in an ON state).
In general, a mechanical circuit is used for the SW circuit 10.
When the SW circuit 10 is turned ON, the output capacitance 24 is charged up to the voltage of the power supply 20 from the power supply 20 through the SW circuit 10, the coil 21, and the diode 23, and a large inrush current flows by this. This is shown in FIGS. 7A and 7B.
In FIGS. 7A and 7B, the horizontal axis indicates time, FIG. 7A shows a current Ip of the power supply 20, and FIG. 7B shows a voltage Vout of the output capacitance 24. At a time T1, the SW circuit 10 is turned ON, and at a time T2, the output capacitance 24 is charged to the voltage of the power supply 20, and the current of the power supply 20 becomes small. Thereafter, a boosting operation is gradually performed by a soft start circuit.
However, in the conventional booster type SW regulator, there is a problem that when the power supply is turned ON from the OFF state (when the SW circuit 10 is turned ON from the OFF state), a large inrush current flows.
The present invention has been made in order to solve the problem described in the related art, and therefore it is an object of the present invention to suppress an inrush current at the time when a power supply of a booster type SW regulator is turned ON.
In order to solve the above-mentioned problem, according to this invention, a MOS transistor is provided between a coil of a booster type SW regulator and a power supply, and when the power supply is turned ON, a resistance value of the MOS transistor is changed from a high state to a low state, so that an inrush current at the time when the power supply is turned ON is suppressed.